Sunday, May 3, 2015

Review on ALD of Metal Sulfides

Tuomo Suntola demonstrated the growth
of ZnS thin films by ALD 40 years ago growing ZnS. This was the starting point of ALD development in Finland
and later ALD research and industrialization of the method worldwide. Today novel applications in energy storage, catalysis, and nanophotonics have lead to an increased interest in metal sulfide materials. The recent focus on 2D layered materials like single-layer MoS2 researched as transistor channel material, is probably the driver in this renewed interest in chalcogenide ALD. Here is a rather fresh review paper on ALD of metal sulfides from University of
Michigan and Argonne National Laboratory.

The
field of nanoscience is delivering increasingly intricate yet elegant
geometric structures incorporating an ever-expanding palette of
materials. Atomic layer deposition (ALD) is a powerful driver of this
field, providing exceptionally conformal coatings spanning the periodic
table and atomic-scale precision independent of substrate geometry. This
versatility is intrinsic to ALD and results from sequential and
self-limiting surface reactions. This characteristic facilitates digital
synthesis, in which the film grows linearly with the number of reaction
cycles. While the majority of ALD processes identified to date produce
metal oxides, novel applications in areas such as energy storage,
catalysis, and nanophotonics are motivating interest in sulfide
materials. Recent progress in ALD of sulfides has expanded the diversity
of accessible materials as well as a more complete understanding of the
unique chalcogenide surface chemistry.

ALD of sulfide materials typically uses metalorganic precursors and hydrogen sulfide (H2S).
As in oxide ALD, the precursor chemistry is critical to controlling
both the film growth and properties including roughness, crystallinity,
and impurity levels. By modification of the precursor sequence,
multicomponent sulfides have been deposited, although challenges remain
because of the higher propensity for cation exchange reactions, greater
diffusion rates, and unintentional annealing of this more labile class
of materials. A deeper understanding of these surface chemical reactions
has been achieved through a combination of in situ studies and
quantum-chemical calculations. As this understanding matures, so does
our ability to deterministically tailor film properties to new
applications and more sophisticated devices.

This
Account highlights the attributes of ALD chemistry that are unique to
metal sulfides and surveys recent applications of these materials in
photovoltaics, energy storage, and photonics. Within each application
space, the benefits and challenges of novel ALD processes are emphasized
and common trends are summarized. We conclude with a perspective on
potential future directions for metal chalcogenide ALD as well as
untapped opportunities. Finally, we consider challenges that must be
addressed prior to implementing ALD metal sulfides into future device
architectures